1. Field of the Invention
The present invention relates to a magnetic recording medium used in a magnetic disk drive and a manufacturing method thereof. More particularly, the present invention relates to a magnetic recording medium which has a large residual magnetization and high coercive force such that a high signal to noise ratio is realized when the disk is used in the magnetic disk drive.
2. Description of the Prior Art
In the prior art, it is known that a structure of a magnetic film cobalt alloy such as a cobalt-nickel (Co-Ni) alloy or cobalt-nickel-chromium (Co-Ni-Cr) alloy, deposited on a chromium base layer formed on a substrate is a suitable magnetic recording medium having high residual magnetization and high coercivity for use in a magnetic disk drive. A cross section thereof is shown in FIG. 1, wherein a substrate 11 of an aluminum alloy or a hard glass is used. A Cr base layer 12, magnetic film 13 of Co-Ni or Co-Ni-Cr alloy, and a cover layer 14 are laminated one by one on the substrate. Cover layer 14 is of carbon (C), silicon oxide or the like, and serves both as a lubricating and a protective layer. Sputtering technology using an argon gas is generally used in forming the above multilayer structure.
Representative values for the above magnetic recording medium using Co-Ni alloy as the magnetic film are shown in Table 1.
TABLE 1 ______________________________________ Coercivity Hc; 600 Oe Residual Magnetization Br; 13000 Gauss Squareness Ratio Sq (Bs/Br); 0.9 Signal to Noise Ratio S/N; 35-38 dB ______________________________________
In Table 1, Bs denotes saturation magnetization, signal level S is measured as a reproduced peak to peak signal voltage for a record of isolated magnetic transitions, and noise N is measured as an rms (root mean square) noise voltage for a recording density of D.sub.50, wherein D.sub.50 is defined as a recording density at which the signal output is reduced to 50% from the level for isolated magnetic transitions.
When the above values are compared with those for a sputtered .gamma.-Fe.sub.2 O.sub.3 magnetic thin film, coercivity Hc is almost the same, but residual magnetization Br values for the former are greatly increased. In the case of .gamma.-Fe.sub.2 O.sub.3, Br values of about 2500 Gauss can be obtained. Therefore, the magnetic recording medium comprising a chromium base layer and a magnetic film of a Co alloy is superior in performance in obtaining a higher output and easy application even for a magnetic disk drive of low speed. On the other hand, S/N ratio values such as 35-38 dB for the Cr base layer and Co-Ni alloy magnetic film are lower than the values obtained for an .gamma.-Fe.sub.2 O.sub.3 magnetic film, which shows a higher S/N ratio such as 46-48 dB.